JP2002270869A - Solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an efficient solar battery by devising the formation form of a silicon nitride film that is formed as a rear surface passivation film by considering the interference between reflection light on the surface of a rear surface metal electrode layer and reflection light at the boundary of a silicon single-crystal substrate and the silicon nitride film. SOLUTION: When no texture mainly composed of 111} face is formed on both the reverse surface MPP and the light reception surface MPS of the silicon single crystal substrate 3 in the rear surface structure of the solar battery 1, the film thickness of the silicon nitride film 4 at the rear surface side is set to 40 nm-220 nm. Also, when texture formation is made only at the side of the light reception surface MPS, the film thickness of the silicon nitride film 4 at the rear surface side is set to 100 nm-300 nm. Further, when texture formation is made on both the rear surface MPP and the light reception surface MPS, the film thickness of the silicon nitride film 4 at the rear surface side is set to 40 nm-230 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell using a silicon single crystal.

[0002]

2. Description of the Related Art In a solar cell using a silicon single crystal, one of the important technical issues is how to efficiently convert irradiated light energy into electric energy. Generally, the surface of a silicon single crystal substrate that has been smoothed by polishing has a high light reflectance, which causes a loss of light energy (hereinafter referred to as reflection loss). On the other hand, a silicon single crystal substrate whose dopant concentration is not so high has relatively high light transmittance, and light incident on the substrate from the light receiving surface (in this specification, the second main surface of the substrate) is applied to the back surface side. When transmitted (in the present specification, it is referred to as the first main surface of the substrate), this also causes a loss of light energy (hereinafter, referred to as the first main surface).
Transmission loss).

[0003] Among them, as an improvement for reducing the reflection loss on the light receiving surface side, there is a method of forming an antireflection film on the light receiving surface side. Recently, however, a silicon single crystal using anisotropic etching has been used. A substrate surface roughening process, a so-called texture process, is often used. In this method, {111} planes having various orientations are preferentially exposed by anisotropically etching the (100) plane of a silicon single crystal using an etching solution such as hydrazine aqueous solution or sodium hydroxide. This is a roughened structure (hereinafter referred to as texture). The incident light is irregularly reflected by the unevenness of the texture, and as a result, the light flux satisfying the light receiving surface and the total reflection condition is reduced.
The efficiency of light incidence on the substrate is increased, and reflection loss on the light receiving surface side can be reduced. The texture can also be formed on the back surface side, whereby the light reflected on the back surface side can be scattered, and the loss due to the light receiving surface going out of the battery again can be suppressed.

On the other hand, as an improvement for reducing transmission loss,
A method has been proposed in which a reflective film is formed on the back surface side, and the light that is going to escape from the back surface is reflected back and returned into the substrate.
As such a reflective film, a metal electrode layer covering the entire back surface (hereinafter, referred to as a back metal electrode layer) is used. However, if the metal electrode layer is brought into direct contact with the main surface of the silicon single crystal substrate, the loss due to surface recombination of photogenerated carriers increases, so the passivation film is provided between the metal electrode layer and the silicon single crystal substrate. For example, a silicon nitride film is formed. In this case, the contact between the metal electrode layer and the underlying silicon single crystal substrate is ensured through a conductive through-hole formed in the silicon nitride film by photolithography or mechanical processing.

[0005]

Here, the light incident from the light receiving surface side is reflected by two surfaces of the back surface metal electrode layer and the boundary between the silicon single crystal substrate and the silicon nitride film. The light reflected by the metal electrode layer is refracted at the boundary and then transmitted through the silicon nitride film once at the time of incidence and once at the time of reflection, resulting in an optical path difference with the boundary reflected light. Cause interference. Conventionally, no technical consideration has been given to the effect of this interference on the conversion efficiency of the solar cell.

An object of the present invention is to provide a method of forming a silicon nitride film to be formed as a back surface passivation film in consideration of interference between reflected light at the back metal electrode layer and light reflected at the silicon single crystal substrate / silicon nitride film boundary. It is intended to realize a solar cell with higher efficiency by devising the solar cell.

[0007]

Means for Solving the Problems and Actions / Effects In order to solve the above problems, a first configuration of a solar cell according to the present invention comprises:
A second main surface of a silicon single crystal substrate having a crystal main axis direction of <100> and having neither a first main surface nor a second main surface having a texture mainly composed of a {111} plane. On the other hand, a silicon nitride film having a thickness of 40 to 220 nm is formed on the first main surface on the back surface side, and a back metal electrode layer is formed so as to cover the silicon nitride film. It is characterized by becoming.

In the second configuration, the crystal main axis direction is <1.
00>, a texture mainly composed of the {111} plane is not formed on the first main surface, and the second main surface of the silicon single crystal substrate on which the texture is formed is received on the second main surface. A silicon nitride film having a thickness of 100 nm to 300 nm is formed on the first main surface on the other side, and a back metal electrode layer is formed so as to cover the silicon nitride film. It is characterized by.

Further, in the third configuration, the crystal main axis direction is <
100>, and the second main surface of the silicon single crystal substrate in which the texture mainly composed of the {111} plane is formed on both the first main surface and the second main surface is defined as a light receiving surface;
On the other hand, a silicon nitride film
It is formed in a thickness of 0 nm to 230 nm, and a back metal electrode layer is formed so as to cover the silicon nitride film.

In the case of a solar cell using a silicon single crystal substrate whose main crystal axis direction is <100>, the thickness of the silicon nitride film formed on the back surface is sufficient to ensure the function as a passivation film. Of course you have to choose The lower limit of this thickness is determined from the viewpoint of suppressing surface recombination of photogenerated carriers due to tunnel current.
It can be estimated to be about 20 nm. However, the present inventors have examined in detail that the formation of the silicon nitride film in a specific film thickness range determined independently of the lower limit value allows the internal reflection efficiency of the back metal electrode layer, and thus the light / They have found that the energy conversion efficiency can be improved, and have completed the present invention.

The specific thickness range of the silicon nitride film depends on whether or not texture is formed on the first main surface (back surface side) and the second main surface (light receiving surface side) of the silicon single crystal substrate. Will be different. The first configuration is a case where no texture is formed on either the first main surface or the second main surface, and the optimum thickness range of the silicon nitride film is 40 to 22.
0 nm. Even if the film thickness exceeds the upper limit or falls below the lower limit of this range, the effect of improving the internal reflection efficiency cannot be expected in any case.

In the second configuration, the texture is formed only on the second main surface side, and the optimum thickness range of the silicon nitride film is 100 to 300 nm. If the film thickness is less than the lower limit of this range, the effect of improving internal reflection efficiency cannot be expected. On the other hand, the upper limit is determined in consideration of the formation time and cost of the silicon nitride film.

A third configuration is a case where texture is formed on both the first main surface and the second main surface, and the optimum thickness range of the silicon nitride film is 40 to 230 nm. Even if the film thickness exceeds the upper limit or falls below the lower limit of this range, the effect of improving the internal reflection efficiency cannot be expected in any case.

As described above, the fact that the silicon nitride film on the back side has an optimum thickness range from the viewpoint of improving the internal reflection efficiency is based on the fact that the backside metal electrode layer, the silicon single crystal substrate / silicon nitride film boundary, (Hereinafter, referred to as metal surface reflected light and film boundary reflected light). That is, when the metal surface reflection light and the film boundary reflection light cause interference in the silicon nitride film, the difference in the refractive index between the silicon single crystal substrate and the silicon nitride film causes
The interference state between the two lights changes depending on the thickness of the silicon nitride film. Then, in a specific film thickness range, a condition for strengthening the metal surface reflected light and the film boundary reflected light is satisfied,
It is considered that the internal reflection efficiency is improved.

[0015]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of the solar cell of the present invention. The solar cell 1 has a crystal main axis of <100>.
The second main surface MPS of the p-type silicon single crystal substrate 3 is used as a light receiving surface side, and an emitter layer 42 in which an n-type dopant is diffused is formed here, thereby forming a pn junction portion 48 and further oxidizing. Film 43, electrode 44 and antireflection film 47
Are formed in this order. The electrode 44 on the light receiving surface side
In order to increase the efficiency of light incidence on the pn junction 48, for example, a finger electrode as shown in FIG.
Thick busbar electrodes are provided at appropriate intervals to reduce internal resistance.

On the other hand, as shown in FIG. 3, a silicon nitride film 4 is formed on the first main surface MPP on the back surface side, and a back metal electrode layer 5 is formed so as to cover the silicon nitride film 4. ing. The back metal electrode layer 5 is the first main surface MPP
And is formed, for example, as an aluminum vapor-deposited layer. The back metal electrode layer 5 is electrically connected to the underlying silicon single crystal substrate 3 (silicon semiconductor layer) through a contact penetrating portion 6 penetrating the silicon nitride film 4 in the thickness direction. Although the contact penetration portion 6 may be formed by photolithography, in the present embodiment,
It is a groove formed by machining or a hole formed by laser processing.

First main surface MPP of silicon single crystal substrate 3
A texture for antireflection can be formed on one or both of the first and second main surfaces MPS. The texture is, for example, a random texture composed of a large number of pyramid-shaped protrusions having an outer surface of a (111) plane, as shown in FIG. However, it is of course possible to omit this texture. The surface roughness of the textured main surface of the silicon single crystal substrate 3 is JIS: B0601 (198
The center line average roughness Ra specified in 2) is 0.5 to 50.
It is about μm. On the other hand, when it is not formed, it is, for example, a chemically polished surface and has a center line average roughness Ra of 0.5 μm or less.

As described above, when the texture is not formed on either the first main surface MPP or the second main surface MPS in FIG. 1, the optimum thickness t range of the silicon nitride film is 40 in FIG. ２２０220 nm. When the texture is formed only on the second main surface MPS side, the optimum thickness range t of the silicon nitride film 3 is 100 to 3
00 nm. Further, when the texture is formed on both the first main surface MPP and the second main surface MPS, the optimum thickness range of the silicon nitride film is 40 to 230 nm.

Hereinafter, the manufacturing process of the solar cell 1 will be described mainly on the back side. However, the present invention is not limited to a solar cell manufactured by this method. First, FIG.
As shown in (a), high-purity silicon is doped with a group III element such as boron or gallium, and the specific resistance is set to 0.1.
An aqueous potassium hydroxide solution is applied to the cut silicon single crystal substrate 3 having a thickness of 5 to 5 Ω · cm by a known method.
Alternatively, after immersing in a sodium hydroxide aqueous solution for a certain period of time to remove the damaged layer, texture formation is performed. The silicon single crystal substrate 3 may be manufactured by any of the CZ method and the FZ method, but is preferably manufactured by the CZ method from the viewpoint of mechanical strength.

After the texture is formed, the silicon single crystal substrate 3 is washed in an acidic aqueous solution of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid or the like, or a mixture thereof. From the viewpoint of economy and efficiency, the silicon single crystal substrate 3 is washed in hydrochloric acid. Is preferred. In some cases, this texture forming step may be omitted. Next, FIG.
As shown in FIG. 1, a silicon nitride film 4 is formed on a first main surface (hereinafter, referred to as a back surface) of a substrate 3 by a known method. The silicon nitride film forming process includes a normal pressure thermal CVD method,
Any method such as a low pressure thermal CVD method and a photo CVD method can be used.
It is preferable to prepare by Method D.

Since the silicon nitride film 4 is also effective as a phosphorus diffusion mask, at this stage, an emitter is formed on the second main surface MPS of the substrate by a vapor phase diffusion method using phosphorus oxychloride. A layer may be formed. In order to enhance the effect as a diffusion mask, it is preferable that the surfaces of the two silicon single crystal substrates 3 on which the silicon nitride films 4 are formed are overlapped, and a pair of the two silicon single crystal substrates is arranged in a diffusion boat to perform gas phase diffusion. And about 850 in a phosphorus oxychloride atmosphere.
C. to form an n-type emitter layer on the second main surface MPS. The depth of the formed emitter layer is about 0.5 μm, and the sheet resistance is 40 to 100Ω / □.

Then, as shown in FIG. 4C, a groove or a hole as a contact penetrating portion is formed on the back surface of the substrate 3. For example, when forming the groove 6, the high-speed rotary blade 1
3 is engraved. High-speed rotary blade has a diameter of 103m
100 to 200 uneven forming blades are attached to a cylindrical portion having a length of 165 mm and a length of 165 mm. The height of the blade is, for example, 50-
100 μm, the width of the blade and the interval between the blades are about several hundred μm. Diamond abrasive grains having a diameter of 5 μm to 10 μm are uniformly electrodeposited on the surface of the blade. This high-speed rotary blade 13
And grooving the substrate at a speed of about 1 to 4 cm per second while spraying a cutting fluid. The high-speed rotary blade 13
A dicer or wire saw can be used instead.
The rotary blade device is finely adjusted so that the depth of the groove 6 is approximately 5 to 50 μm.

On the other hand, when a hole is formed instead of the groove 6,
Laser beams are preferably used. As the laser, a carbon dioxide laser, an argon laser, a YAG laser, a ruby laser, an excimer laser, or the like is easily used. Among them, excimer laser such as KrF and Nd:
A YAG laser is optimal. The plane shape of the hole is circular,
An oval or rectangular shape can be adopted. Note that the laser irradiation conditions for providing the opening are appropriately determined depending on the type of laser, the thickness of the insulating layer, the diameter of the opening, and the like. For example, when using pulse oscillation, the frequency is 1H
z to 100 kHz is preferable, and the average output of the laser is preferably in the range of 10 mW to 1 kW.

After forming the contact penetrating portions such as the grooves 6 and the holes,
As shown in FIG. 4D, a back metal electrode layer 5 is formed on the same surface (first main surface side), for example, in a thickness of 0.5 to 2 μm. Although metals such as silver and copper can be used for the electrodes,
Aluminum is most preferable from the viewpoint of workability. The metal layer can be deposited by any method such as a sputtering method, a vacuum evaporation method, and a screen printing method. The back metal electrode layer 5 is uniformly deposited on the first main surface MPP while filling the contact through portions.

Thereafter, the antireflection film 47 and the electrode 44 on the second main surface MPS (light receiving surface) side of FIG.
Is formed. Silicon oxide,
In addition to silicon nitride, cerium oxide, alumina, tin dioxide, titanium dioxide, magnesium fluoride, tantalum oxide, and the like, and a two-layer film obtained by combining two kinds of these are used, and any of them can be used without any problem. The film is formed by PVD
Method, a CVD method, or the like is used, and any method is possible. For manufacturing a high-efficiency solar cell, silicon nitride formed by a remote plasma CVD method is preferable because a small surface recombination rate can be achieved. On the other hand, electrode 4
4 is manufactured by a vapor deposition method, a plating method, a printing method, or the like. Either method may be used, but a printing method is preferable for low cost and high throughput. For example, an electrode pattern is screen-printed using a silver paste in which silver powder and glass frit are mixed with an organic binder, and then heat-treated to form an electrode. Note that there is no problem even if the order of the processing on the first main surface MPP side and the processing on the second main surface MPS side are reversed.

The thickness of the silicon nitride film 4 on the first main surface MPP side is adjusted to each of the aforementioned optimum film thickness ranges depending on whether or not a texture is formed on the silicon single crystal substrate 3. The validity of this film thickness range is supported not only by the experimental results described later but also by the calculation results based on the following refraction / reflection theory. First, the reflectance of light incident from the second main surface MPS side on the first main surface MPP side is calculated by refraction / reflection theory. The outline is as follows. First,
The laws of reflection and refraction of light propagating in two different media, that is, electromagnetic waves, are uniquely defined by the Maxwell's equations from the boundary conditions of electric flux density, magnetic flux density, electric field and magnetic field to be satisfied at the boundary of the medium. Can be derived.
The derivation process is described in a general textbook on optics or electromagnetics, and is very well known.
Detailed description is omitted (for example, Asakura Modern Physics Course 2:
Electromagnetics I (Hisashi Matsuda: 1980: Asakura Shoten) 63
Pp. 68). As a result, if the incident angle, refraction angle, and reflection angle of light are θ, β, and γ, respectively, then the law of reflection is θ = γ 、, and the law of refraction (so-called Snell's law) is , Sin θ / sin β = n2 / n1 ‥‥. The medium on the light incident side (in this case, silicon single crystal) with the boundary therebetween as the medium 1 and the medium on the transmission side (in this case, silicon nitride) as the medium 2, and n 1 and n 2 are the medium 1 and the medium 1. This is the refractive index of the medium 2. The refractive index n1 of silicon single crystal is 3.52, and the refractive index n2 of silicon nitride is 2.00.

By applying the above Snell's law to the well-known Fresnel's formula and calculating the intensity of incident light and reflected light (which can be expressed as either an electric field or a magnetic field), the silicon single crystal substrate 3 The reflectance of the boundary reflection with the silicon nitride film 4 can be obtained. On the other hand, the back metal electrode layer 5
It is thought that the influence of light penetration and refraction on the back metal electrode layer 5 side at the boundary between the silicon nitride film 4 and the silicon nitride film 4 is small.
Here, fine adjustment was made by introducing a complex refractive index (absorption coefficient).

FIG. 6 shows how the reflectance of the boundary reflection between the silicon single crystal substrate 3 and the silicon nitride film 4 changes according to the thickness of the silicon nitride film using the incident angle θ as a parameter. (The incident wavelength is 1200 n
m). If θ is equal to or less than the critical angle of total reflection of 34.6 °, a maximum value occurs in the reflectance due to an interference effect with light reflected from the metal surface. Then, part of the long-wavelength light reaching the second main surface MPS is returned to the inside of the silicon single crystal substrate 3 again with this reflectance. Next, FIG. 7 shows the result of calculating the dependency of the photocarrier generation ratio on the internal reflectance using the thickness of the silicon single crystal substrate 3 as a parameter. The effect of the internal reflectance is more remarkable as the substrate thickness is smaller. When the internal reflectance exceeds 93%, the generation ratio of photocarriers increases rapidly. That is, for a high-output solar cell, a structure having an internal reflectivity of 93% or more may be provided on the back surface of the solar cell, and the corresponding silicon nitride film thickness can be obtained from FIG. In other words, in the case of a silicon solar cell having no texture, it can be understood that θ = 0 ° in FIG. 6 corresponds to a silicon nitride film thickness of 40 to 220 nm.

Next, consider the case where there is a texture on the second main surface (light receiving surface) MPS side. The texture is formed using the difference in the etching rate between the {100} plane and the {111} plane of the silicon single crystal, and as shown in FIG. 5, a regular pyramid having a size of several μm to several tens μm. The structure is formed randomly. Since this structure is composed of an equivalent {111} plane of silicon single crystal, the angle α formed by the slope of the pyramid with the second main surface MPS is 54.7 °. On the other hand, since the refractive index of silicon is 3.52 at a long wavelength, using the Snell's law, the incident angle θ = 4.
The incidence on the back side at an angle of 1.3 ° is derived from a simple calculation. Therefore, the second main surface (light receiving surface) MPS
The optimum silicon nitride film thickness range of a solar cell having a texture and a flat first main surface (rear surface) MPP is θ in FIG.
= 41.3 °, which indicates that it is sufficient if the thickness is approximately 100 nm or more.

Finally, when the texture is provided on both surfaces, the pyramids of the texture of the second main surface (light receiving surface) MPS are randomly arranged, so that the pyramids are uniformly dispersed, and light is dispersed on the first main surface (back surface). 41.3 ° incident angle on MPP
Suppose that the incident light is uniform. In each of the pyramids of the texture of the first main surface MPP, 55.4% of the incident light is incident at an incident angle of 13.4 ° and 44.6% is incident at 64.3 °. It can be calculated by considering a simple square pyramid that stands upright. From FIG. 6, θ = 13.4 °
It can be seen that the thickness of silicon nitride in which both θ and 64.3 ° exceed 93% is in the range of approximately 40 to 230 nm.

[0031]

(Example 1) A p-type silicon single crystal substrate having a thickness of 150 μm and using boron as a dopant (crystal main axis direction <100>: cut-up state: specific resistance 1 Ω · cm) was used.
The sheets were immersed in an aqueous solution of potassium hydroxide to form a texture on both sides. Next, a silicon nitride film was formed on the first main surface (rear surface) to a thickness of 150 nm and 300 nm, respectively, and then a parallel groove was formed as a contact penetrating portion using a commercially available dicer. Aluminum was deposited on the entire surface to form a back metal electrode layer. On the other hand, on the second main surface (light receiving surface), an emitter layer, an antireflection film, a finger electrode, and a bus bar electrode were sequentially formed by the above-described method, to produce a solar cell test product.

Using a solar simulator (YSS-80) manufactured by Yamashita Electric Equipment Co., Ltd., the obtained solar cell test product was measured for the IV characteristics of these solar cells under standard conditions to determine the conversion efficiency. Table 1 shows the results.

[0033]

[Table 1]

It can be seen that in the solar cell having a silicon nitride film thickness of 150 nm, the short-circuit current is increased and the conversion efficiency is higher than in the case of 300 nm.

(Example 2) A solar cell test sample produced in exactly the same manner as in Example 1 except that a silicon single crystal substrate having no texture on both sides was used, and the same measurement was performed. Table 2 shows the results.

[0036]

[Table 2]

A solar cell having a silicon nitride thickness of 150 nm
It can be seen that the conversion efficiency is higher than that of 300 nm.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view illustrating an example of a solar cell of the present invention.

FIG. 2 is a perspective view showing an example of an electrode formation form on a light receiving surface side.

FIG. 3 is a schematic cross-sectional view showing, on an enlarged scale, a structure on the back surface side of the solar cell of FIG. 1;

FIG. 4 is an explanatory view showing a step of forming the back surface structure of FIG. 3;

FIG. 5 is a perspective view showing an example of a texture form.

FIG. 6 is a view showing a calculation result of a relationship between a back surface internal reflectance at various incident angles and a silicon nitride film thickness.

FIG. 7 is a diagram showing a result of calculating a relationship between a back surface internal reflectance and a photocarrier generation ratio for various thicknesses of a silicon single crystal substrate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solar cell MPP 1st main surface MPS 2nd main surface 4 Silicon nitride film 5 Back metal electrode layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Otsuka 2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu Semiconductor Semiconductor Isobe Research Laboratory (72) Inventor Masatoshi Takahashi 2-chome, Isobe, Annaka-shi, Gunma 13-1 Shin-Etsu Semiconductor Co., Ltd. Semiconductor Isobe Laboratory (72) Inventor Toshiyuki Ikushima 2-3-1 Isobe, Annaka-shi, Gunma Shin-Etsu Semiconductor Co., Ltd. Semiconductor Isobe Laboratory F-term (reference) 5F051 AA02 CB27 FA06 FA15 FA18 FA23

Claims (3)

[Claims] 1. The crystal main axis direction is <100>, and both the first main surface and the second main surface have {111}.
In the silicon single crystal substrate in which the texture mainly composed of a surface is not formed, the second main surface is used as a light receiving surface, and on the other hand, a silicon nitride film is 40 to
A solar cell having a thickness of 220 nm and a back metal electrode layer formed so as to cover the silicon nitride film. 2. The crystal main axis direction is <100>, and a texture mainly composed of {111} planes is not formed on a first main surface, and the silicon on which the texture is formed is formed on a second main surface. A silicon nitride film having a thickness of 100 nm to 300 nm is formed on the first main surface, which is a back surface side, of the single crystal substrate;
A solar cell, further comprising a back metal electrode layer formed so as to cover the silicon nitride film. 3. The crystal main axis direction is <100>, and both the first main surface and the second main surface have {111}.
The second main surface of the silicon single crystal substrate on which a texture mainly composed of a surface is formed is a light receiving surface, and a silicon nitride film is 40 nm to 2 nm on the first main surface on the back surface side.
A solar cell having a thickness of 30 nm and a back metal electrode layer formed so as to cover the silicon nitride film.